<!DOCTYPE html>
<html lang="zh">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .tudou {
            position: relative;
            width: 444px;
            height: 320px;
            margin: 100px auto;
        }
        .tudou  img{
            width: 444px;
            height: 320px;
        }
        /* content---可以为空，但是不能不写 */
        .tudou::before {
            display: none;
            content: '';
            position: absolute;
            top: 0;
            left: 0;
            width: 100%;
            height: 100%;
            background: rgba(0, 0, 0, 0.3) url(images/arr.png) no-repeat center;
        }
        /* 鼠标经过.tudou就让::before显示出来 */
        .tudou:hover::before {
            display: block;
        }
    </style>
</head>
<body>
    <div class="tudou"><img src="images\tudou.jpg" alt=""></div>
    <div class="tudou"><img src="images\tudou.jpg" alt=""></div>
    <div class="tudou"><img src="images\tudou.jpg" alt=""></div>
    <div class="tudou"><img src="images\tudou.jpg" alt=""></div>
</body>
</html>